(1) Field of the Invention
This invention relates to coated abrasives and to the method of their manufacture. More specifically, the invention is concerned with a finished cloth particularly suitable for use as a backing in a coated abrasive, and in its method of manufacture.
(2) Description of the Prior Art
Basically, a coated abrasive comprises a backing, a maker adhesive to secure abrasive grain to the backing, and a sand size or adhesive layer to more firmly secure the abrasive grains in position.
A backing suitable for use in a coated abrasive must exhibit not only good adhesion with the maker adhesive but must also resist migration of the maker adhesive. If the maker adhesive migrates into the backing on which it is coated, there results a backing having physical properties similar to those of the maker adhesive. Thus, in the case of a heat-hardenable resin maker adhesive, migration results in the backing becoming stiff and hard, thereby destroying to a certain degree the natural pliability of the backing. Such embrittlement of the backing contributes to its failure in use, and moreover results in relatively low tear strength in the backing because of lack of mobility of the individual yarns.
Pliability, tear strength, and the like physical properties are valuable features in a coated abrasive; therefore, any means or method used to improve such physical properties is of distinct importance and advantage and constitutes an improvement in the art.
Heretofore, numerous means have been employed for producing coated abrasives of improved physical properties, none of which have met with universal success. One such means involves the provision of an immiscible, flexible barrier coat on the backing onto which is applied the heat-hardenable maker adhesive. Others have suggested, among other things, lighter-weight backings and the addition of plasticizers to the maker adhesive. Filling a cloth backing with a starch-base material, and treating such filled cloth with a very thin and "green" synthetic resin in preparation to coating the thus-treated cloth with a heat-hardenable "maker" coat is disclosed in U.S. Pat. No. 2,805,136.
Current practice in essentially all cloth finishing of backings for coated abrasives generally involves treatment of the cloth backing with aqueous dispersions of various kinds of filling material, such as starch, glue or clay. Combination treatments, often involving one or more back filling, back sizing, front sizing, etc. operations, are deemed important and necessary operations when a relatively rigid, hard and brittle maker adhesive is employed to secure the abrasive grains to the backing. A primary purpose of such finishing operations is to prevent migration of the maker adhesive into the backing, thus preventing embrittlement of the same. Embrittlement, as before mentioned, adversely affects pliability and tear strength of the coated abrasive. A secondary consideration in preventing migration of maker adhesive is anchorage of abrasive grain. Migration can result in depletion of the amount of adhesive in the maker layer to such an extent that anchorage of the abrasive grains to the backing is adversely affected.
Various methods and apparatus are used in finishing cloth, i.e., in back filling, back and front sizing. Two methods commonly used involve using a "Tommy Dodd" and a two-roll padder. Using a "Tommy Dodd", the cloth backing is forwarded on the surface of a roll partially immersed in a solution of the back filling material. The cloth on immersion is saturated by and picks up the solution in the interstices of the cloth and on its outer surface. On emergence from the vessel containing the back fill solution, the wet cloth, still on the roll, is passed through a narrow opening formed by a sharp-edged blade positioned adjacent to and at an angle with respect to the roll surface. The back fill material is more or less "pounded" into the cloth backing by virtue of the blade pressing against it. In a two-roll padder, the cloth backing is passed through the nip formed by, e.g., two vertically disposed rolls. The back size, for example, is picked up on the surface of the bottom roll which rotates in a tank containing a solution of the size, and is forwarded to the nip formed by the rolls. At the nip, the size is pushed or pounded, by the applicator roll, into the cloth backing.
Such finishing procedures have been selected in the past because, by the very nature of such, the finishing material must necessarily go into and fill up the openings and interstices in the cloth backing, thus leaving no place to which the maker can migrate. Moreover, to ensure more or less complete impregnation by the finishing materials, it has been common practice in the past to dry the coating materials by passing the wet coated surface in contact with a steam-heated drum, can, or the like. Such manner of drying results, as one might expect, in further and more complete migration of the filling and/or size material into the yarns and interstices of the cloth backing. With such finishing operations, the interstices of the cloth and the yarns are generally completely filled.
The finishing operations as above-mentioned are time-consuming and add materially to the cost of manufacture of the coated abrasive. Moreover, although the main object of such finishing operations is accomplished, i.e., migration of the maker adhesive is prevented, such finished cloth backing is attendant with other disadvantages.
Back filling, for example, while contributing to generally higher tensile properties, as compared with unfilled cloth, adversely affects, at least initially, tear strength and pliability, hence the over-all performance of the coated abrasive. On the other hand, a back fill is believed to have little influence on the physical properties of a coated abrasive once it has been in use for a relatively short period of time. Thus, back filling, it is believed, offers no real advantages and therefore ties up men and machinery unnecessarily. The use of a heat-hardenable resin size, however, which itself is allowed to migrate into and fill the backing, instead of the resin maker adhesive, makes for embrittlement of the backing and, it is believed, continually during the life of the coated abrasive affects pliability and tear strength in an adverse manner.